1. Field of the Invention
The present invention relates to a photographic lens system used in a so-called compact camera.
2. Related Background Art
A so-called telephoto-type wide angle lens system comprising a front group having a positive refracting power and a rear group having a negative refracting power can be made shorter in the overall length than a Tessar type lens system and used as a photographic lens system of a compact camera. The lens system of the type in which the front group consists of a positive lens, a negative lens and another positive lens (the total being three lenses) and the rear group consists of a single negative meniscus lens is manufactured at a relative low cost and aberrations can be corrected well. Thus, the lens system of this type is widely used. The structure of this lens system is disclosed in U.S. Pat. Nos. 4,303,313 and 4,413,888, for example.
However, very few telephoto-type lens systems had large apertures of an F-number of 2 or less. In particular, the smallest F-number of the telephoto-type lens system comprising the four groups consisting of four lenses as above-mentioned was considered to be 2.8. For example, the lens system disclosed in U.S. Pat. No. 4,303,313 has an F-number of about 3.5 and thus cannot be said that it has a large aperture. U.S. Pat. No. 4,413,888 only achieved an F-number of 2.8.
Later, U.S. Pat. No. 4,787,721 disclosed a lens system which intends to provide a large aperture. However, the lens system proposed by this patent must consist of five lenses and more having aspherical surfaces and has left problems on cost and productivity.